Shigella, a Category B agent, is responsible for an estimated 1.1 million deaths annually. An in-depth understanding of the pathogen-host interaction is essential to the treatment and development of new therapeutics against diseases. Shigella pathogenesis involves bacterial invasion of the colonic epithelium and subsequent intercellular spread. One aspect of this process that is poorly understood is the spread of Shigella from one cell into an adjacent uninfected cell. I propose the first genome-wide analysis of host cell factors required for S. flexneri pathogenesis, with a particular focus on the factors required for S. flexneri intercellular spread. My Specific Aims are: 1) to identify host factors necessary for S. flexneri spread via a genome-wide siRNA screen, 2) to characterize the pathways engaged by S. flexneri during intercellular spread. My approach is highly likely to identify novel factors in the mammalian cell that contribute to S. flexneri pathogenesis, and perhaps the pathogenesis of other Category A-C pathogens. Furthermore, I expect that a screen for small molecule inhibitors of S. flexneri spread currently being carried out in the Goldberg laboratory will generate complementary data, which together with the data generated here, might allow us both to identify small molecule inhibitors of specific pathways critical to S. flexneri and to characterize the mechanism of action of these molecules. These approaches may lead to the development of therapeutic agents for the treatment of Shigella infection. Lay summary: The intestinal bacterium Shigella causes 1.1 million deaths annually and is a potential agent of bioterrorism. A better understanding of the methods by which Shigella causes disease allows the development of preventative and therapeutic agents against Shigella. I propose a large-scale analysis of human proteins that participate in Shigella disease and a detailed investigation of the method in which Shigella interacts with these proteins. [unreadable] [unreadable] [unreadable]